In general, a quantum-cascade laser and a resonant tunneling diode (hereinafter referred to as an “RTD”) are used as an active layer of a current injection oscillator for generating terahertz waves (having a frequency of 30 GHz or higher and 30 THz or lower in the preset specification). A quantum-cascade laser and an RTD provide electromagnetic wave oscillation on the basis of intersubband transition of an electron in a quantum well structure of a semiconductor.
In particular, PTL 1 describes a device using a double-barrier RTD as an active layer for oscillating terahertz waves in a frequency band near 1 THz at room temperature. The double-barrier RTD includes an InGaAs quantum well layer and an InAlAs tunnel barrier layer, which are epitaxially grown on InP. The RTD generates electromagnetic waves using simulated emission in the range of a negative differential resistance appearing in the current-voltage characteristic. In addition, NPL 1 describes a structure that varies a frequency of a center frequency of 470 GHz and a variable width of 18 GHz by varying the driving voltage within the range in which the negative differential resistance appears.